The fruit (pod, nut) of the peanut (Arachis hypogaea L.) is made of an external shell (or hull) (21-29%) and of the nut itself (79-71%), which consists of a thin hull ("skin", seed coat) (2-3%), the kernel (69-73%) and the germ (2.0-3.5%) (van Doosselaere, 2013). The term "peanut" may refer to the whole fruit (including the shell), to the kernel with its thin coat, or to the kernel without the thin coat. Peanuts are nutrient- and energy-rich products that are mainly used for food, but whole cull peanuts or decorticated peanuts are occasionally sold for feed. Like other peanut products, peanuts can be contaminated by aflatoxins and should be tested before being fed to livestock.

Morphology and cultivation

Peanut is an annual herbaceous plant growing to a height of 30 to 50 cm. The peanut plant can be erect or prostrate with a well developed taproot and many lateral roots and nodules. The leaves are opposite and pinnate with four leaflets; each leaflet is 1 to 7 cm long and 1 to 3 cm across. The flowers are 1.0 to 1.5 cm across, bright yellow or yellowish orange with reddish veining. They are borne in axillary clusters on the stems above ground and last for just one day. One to several flowers may be present at each node and are usually more abundant at lower nodes. The first flowers appear at 4 to 6 weeks after planting and maximum flower production occurs 6 to 10 weeks after planting. 8 to 14 days after pollination, a short stalk at the base of the ovary elongates to form a thread-like structure known as a "peg". This pushes the ovary down 5 to 8 cm into the soil, where it develops into a pod. Pods are 3 to 7 cm long, normally containing one to four seeds. Pods reach maximum size after 2 to 3 weeks in the soil, maximum oil content in 6 to 7 weeks, and maximum protein content after 5 to 8 weeks. The peanut crop matures after 7 to 9 weeks in the soil, maturity being measured by maximum levels of protein, oil, dry matter, and the presence of darkened veining and brown splotching inside the pod. Peanuts usually require a minimum of 100 to 150 days from planting to maturity depending on the variety. Flowering continues over a long period, and pods are at all stages of development at harvest. Pegs will eventually rot in the soil and the resulting loose pods are lost during harvest (Putnam, 1991).

Uses

About 41% of the world peanut crop is used for oil production, whereas 45% is used directly as human food (Fletcher et al., 2016). Peanut kernels, usually cooked or toasted, are appreciated worldwide as a flavourful snack food, nutritionally dense due to its high energy, protein and fat content. Peanuts are also the primary ingredient of many finished products such as peanut butter, confections, nutritional bars, and are used in numerous dishes (Davis et al., 2016). Peanuts are usually too valuable to be used as animal feed. However, whole or shelled peanuts, or even roasted peanuts are sometimes fed to livestock. In 2001, for instance, raw peanuts, in excess of the peanut quota in the USA, were sold on the feed market. Likewise, abnormal peanuts, or peanuts with a higher aflatoxin content than that permitted by the food regulations, have been used as feed ingredients for cattle. Peanuts meant for oil production, or for the manufacture of confectionary or peanut butter may also be available for livestock (Myer et al., 2001; Hill, 2002). In the USA, it has been estimated that less than 1% of the annual production is fed as raw peanuts to cattle (Hill, 2002).

Peanut production

Distribution

Peanut is a native from South America. It was cultivated in Peru at least as early as 1500 BCE and probably earlier. The Incas already used the seeds, which were eaten toasted, and the oil. After the arrival of Europeans in America, peanut was spread worldwide. An oil mill was established in Spain in 1800, and West Africa became the primary source of peanut exportation in the 19th century (Pattee, 2005). Peanut is a now a major crop widely distributed throughout tropical, subtropical, and warm temperate areas in Asia, Africa, Oceania, North and South America, and Europe (Freeman et al., 1999). In 2014, peanut cultivation covered 25.7 million ha worldwide, including 13.1 million ha in Africa (51%), 11.2 million ha in Asia (44%) and 1.3 million ha in the Americas (5%) (FAO, 2016).

Peanut is cultivated throughout Africa, Sudan, Nigeria and Senegal being the main producers. In Western and Central Africa, peanut is cultivated in semi-arid areas: the growing season is 75-150 days, with an annual rainfall of 300-1200 mm. In Southern and Eastern Africa, peanut is also grown at altitudes above 1500 m with rainfalls of 300-1000 mm.

In Asia, most of the production occurs in India and China, though peanut is also grown in Indonesia, Myanmar, Bangladesh, and Vietnam. In India, about 80% of the peanut area is rainfed, and it is grown in Southern, Western, and parts of Central India during the south-west monsoon. The remaining 20% is irrigated, and grown in the post-rainy season and summer in Southern, Eastern and Central India. In China, peanut is grown in rotation with wheat and maize. 25% of the production comes from Shandong province in Northern China. Growing conditions are diverse: rainfall ranges from 400 to 2000 mm, and there are 150 to 300 frost-free days per year.

Argentina and Brazil together account for 66% of the peanut produced in Latin America and the Caribbean. The crop is grown mainly in semi-arid regions.

In the USA, production is concentrated in the South-East, in Virginia and Carolina, and the South-West.

In Europe, peanut is grown only in Bulgaria and some areas of Greece, Spain, and Yugoslavia (Freeman et al., 1999).

Even though crushing peanuts for oil and meal remains a major use for peanut production, direct utilisation for food has been steadily increasing since the 1970s. About 45% of the world peanut production was used for food in 2010-2013. 60% or above goes to the food markets of North America, Southern Africa, West Africa and South-Eastern Asia, and only 41% in Eastern Asia and 13% in South-Western Asia (Fletcher et al., 2016). The worldwide production of peanuts (with shells) was 40 million tons in 2015. 40% of this was produced in China, 19% in the other Asian countries, 18% in Africa and 11% in the Americas (USDA, 2016). Assuming that 45% goes to food, it can be estimated that 18 million t of peanuts are produced for the food market.

Low-input systems. In most countries in Africa and Asia, peanut is grown by smallholder farmers as a semi-subsistence crop, grown primarily for food, but small quantities are sold for cash after meeting household consumption requirements. The crop is cultivated under rainfed conditions, with no inputs other than land and labour. It is subjected to drought stress and to high levels of pest and disease infestation. Average yields are about 700 kg/ha and can vary substantially from year to year.

High-input systems. In the USA, Australia, Argentina, Brazil, China, and South Africa peanut is produced on a commercial scale using improved varieties, modern crop management practices, irrigation, and high levels of inputs such as fertilizer, herbicides, and pesticides. Farm operations are generally mechanized. Yields in these systems are considerably higher (2-4 t/ha) and more stable than in low-input systems.

Peanut kernels are a rich source of lipids (and therefore energy) as they contain about 45-50% oil, and are also a rich source of protein (26-34%). Kernel composition can vary substantially depending on specific market requirements, including cultivar, growing environment, storage conditions, maturity, kernel size, and specific thermal processing conditions. Peanut protein is deficient in lysine, methionine and threonine and diets may need specific amino acid supplementation. Oleic acid (C18:1), linoleic acid (C18:2) and palmitic acid (C16:0) account for more than 90% of the fatty acids of peanut oil (Davis et al., 2016). Peanut oil contains about 50% oleic acid and 30% linoleic acid but there are large variations in their respective proportions: reported values for oleic acid range from 35 to 82% and values for linoleic acid from 3 to 43%, due to natural variations and the existence of oleic-rich (and linoleic-poor) varieties (Davis et al., 2016; Pattee, 2005). Peanut kernels contain minimum amounts of fibre, and can be a good source of calcium and vitamin E (Davis et al., 2016).

Cull peanuts used whole contain the fibrous shells, so the fibre content of the pod can be high (ADF up to 26% of DM), and is always variable. Protein and lipid contents are lower than for the dehulled kernels (Myer et al., 2009b).

Potential constraints

Aflatoxins

Peanuts are particularly vulnerable to contamination by fungi Aspergillus flavus and Aspergillus parasiticus. These fungi produce aflatoxins, which are known to cause cancers in humans, increase incidents of hepatitis viruses B and C, lower the immune response system, impair growth in children and cause childhood cirrhosis. In poultry and livestock, aflatoxins can cause loss of appetite, loss of weight, reduced egg production, and contamination of milk (ICRISAT, 2016). Aflatoxin contamination may occur in the field, after peanuts are lifted but before harvest, during transport, and during storage. Either elevated temperatures or drought stress alone increased aflatoxin production, but high temperatures appeared to have a greater impact. Key requirements of post-harvest management to control aflatoxins are the level of seed moisture and insect infestations. Both A. flavus and A. parasiticus are xerophiles that can grow and produce aflatoxin at relative moisture around 85%. Storage below this moisture level or at 10°C will prevent aflatoxin production in storage (Payne, 2016).

In 1960, aflatoxin-contaminated peanut meal from Brazil fed to poultry killed thousands of turkey poults in the UK, drawing attention to these contaminants (FAO, 1979). New regulatory measures in importer countries, such as the EU member states, led to a significant restriction of the trade in peanut and peanut products, and particularly in those produced in sub-Saharan Africa (Waliyar et al., 2007; ICRISAT, 2016). As of 2016, the maximum authorised content in the EU for aflatoxin B1 in feed materials is 0.02 mg/kg (20 ppb or µg/kg) (European Commission, 2003).

Since the 1980s, the reduction of aflatoxins in peanut products has been the subject of considerable research focused on the following areas:

Increase awareness of local populations about aflatoxin issues

Implementation of pre-harvest, post-harvest and storage practices, and technologies for mitigating contamination

Identification/development and use of resistant peanut cultivars

Biological control agents

Methods for detoxifying peanut products.

The objective of aflatoxin management in producing countries is, on the one hand, to limit the aflatoxin exposure in their population, and, on the other hand, to increase exports by decreasing the aflatoxin content of peanut products to a level that meets international requirements (Waliyar et al., 2007; ICRISAT, 2016). As of 2016, aflatoxin contamination in peanut production in tropical countries, and particularly in low-input systems, still remains a major problem, as shown by surveys conducted in China (Wu et al., 2016), Zambia (Bumbangi et al., 2016), Ethiopia (Chala et al., 2016), Nigeria (Ezekiel et al., 2013), Cameroon (Kana et al., 2013), India (Kolhe, 2016a) and Brazil (Oliveira et al., 2009). Aflatoxin contamination of peanut meal in these surveys occasionally reached 100%, with values well beyond international standards.

Progress in breeding for aflatoxin resistance has been limited (Holbrook et al., 2016). Biological control using non-aflatoxigenic strains of Aspergillus has been shown to be effective in Argentina, Australia, the Philippines, and the USA, with reductions ranging from 85% to 98% (Payne, 2016). In Mali, a combined approach using resistant and tolerant cultivars, the application of lime and manure, and better harvesting and drying techniques, helped to reduce aflatoxins by 80-94% (ICRISAT, 2016).

Antinutritional factors

Like other legume seeds, peanuts contain substances with potentially antinutritional effects, such as tannins which are present in the seed coats (Sanders, 1979), lectins, and trypsin inhibitors (Ahmed, 1986; Ahmed et al., 1988a; Ahmed et al., 1988a). These substances have received little attention, perhaps because the antinutritional factors in peanut seem less deleterious than those of other legumes. For instance, a comparison of soybean and peanut flour in rats showed that raw peanut flour was much better tolerated than raw soybean flour, even though lectin concentration and antitrypsin activity were similar in both seeds (Sitren et al., 1985). Peanut lectins can be fully inactivated by heat (moist heat being more effective than dry heat) (Ahmed, 1986) so it is possible that the normal conditions involved in peanut seed and meal processing are enough to make peanut products safe for animal feeding. However, tannins may be a contributing factor for the low protein digestibility of peanut meals (Chiba, 2001).

Ruminants

Information regarding the use of peanuts in ruminants is limited. Trials conducted in Florida and Japan concluded that whole, raw peanuts may have potential as an energy and protein supplement for mature beef cows (Myer et al., 2009a; Myer et al., 2009b; Saito et al., 2016).

Palatability

In the Florida trials, the peanuts were readily consumed by beef cows, but it took several hours for total consumption. In an earlier study, growing heifers did not readily consume whole peanuts, and the peanuts had to be blended with maize grain to encourage consumption (Myer et al., 2009b).

Goats

In Japan, Shiba goats fed a diet containing 15% peanut kernels in the diet had a very high digestibility of lipids (83%), which could be partially attributed to the large proportion of unsaturated fatty acids, resulting in a high content in digestible nutrients. It was concluded that peanuts could be used as a high-energy and high-protein diet for ruminants (Saito et al., 2016).

Pigs

Whole peanuts can be a source of fat, protein and energy for pigs. However, attention should be paid to the low concentration in lysine and sulphur-containing amino acids, to the high content of unsaturated fatty acids, and to potential contamination by aflatoxins.

Growing pigs

Peanuts have been found as effective as added fat in improving feed efficiency, and peanut protein can be utilized efficiently when substituted for other protein supplements on a lysine basis. Maximum inclusion has been suggested to be 10% for diets of growing and finishing pigs (Newton et al., 1990). The inclusion of 5% roasted peanuts was the optimal inclusion rate for weanling pigs (Haydon et al., 1987). The inclusion of 5% raw or roasted peanuts on an equal lysine basis (Newton et al., 1988), or the addition of 10%, 15%, or 20% peanuts (Balogun et al., 1979b) had no effect on growth or nutrient digestibilities in growing pigs. Heat processing of whole peanuts reduced trypsin inhibitor activity, but it had no effect on pig performance (Balogun et al., 1979a). Pigs fed 20% peanuts in the diet had inferior carcass quality (soft fat) compared with those fed the soybean meal diet (Balogun et al., 1979b).

Sows

Peanuts are an excellent source of fat for sow diets. 12% raw and roasted peanuts in the diet could be substituted for 5% animal fat for lactating sows without any effect on reproductive performance (Haydon et al., 1990).

Hogging

Hogging, or hogging-off, is the practice of allowing pigs to feed on peanuts, which they root out of the soil when turned loose in the field. Hogging is not only used for cleaning up waste peanuts after harvesting, but fields have been planted for this purpose as a very cheap method of feeding fattening pigs. This practice used to be common in the USA before the industrialisation of both peanut and pig production: in 1939, 25% of the peanut acreage in Georgia was used for hogging. In 2002, hogging-off had almost disappeared and only 3% of the acreage was grown specifically for pig production (Hill, 2002; Thacker et al., 1992). It is necessary to supply the pigs with a mineral mixture that is high in salt and calcium. Ham from peanut-fed pigs is softer and juicier than normal ham. This can be counteracted, however, by finishing the pigs on a ration containing cottonseed meal, which has the opposite effect on pigs to peanuts (Sheely et al., 1942; Göhl, 1982).

Poultry

Information about the use of peanuts in poultry is limited. Peanut seeds are supposed to have a good nutritive value, with a high energy content provided by lipids, but their use as feed is not economically competitive. Two trials in the USA suggested that raw peanuts could be used as a partial source of protein in broiler diets. Broilers which were fed diets containing 12.5 or 25% raw peanuts had a heavier weight (4 weeks) and a better feed efficiency than those fed peanut meal, but their weight was lower than for the birds fed the maize and soybean control diet. In another trial, broilers fed a diet containing 10% raw peanuts were heavier than those fed the control diet, but there was no significant difference between the control diet and the diets containing 5%, 12.5% and 25% raw peanuts (Offiong et al., 1974).

Rabbits

No information is available in the international literature on the use of peanut seeds in rabbit feeding (2016). Nevertheless, any utilisation of this feed for rabbits, including experimental studies, should take into account the deleterious effects of aflatoxin contamination in rabbits, as described for the peanut meal.